The invention relates generally to a switch substrate and a method for manufacturing the substrate and more particularly to a switch substrate for a contract type switching mechanism for detecting the rotation and position of a counter, such as for detecting the position of a character in a printer.
Several types of switch substrates have been proposed which include a substrate having alternating conducting and insulating portions and a contact brush that passes over the alternating portions in a contact path to detect rotation of the substrate. One such conventional switch substrate 120 is shown in FIG. 5. Substrate 120 is formed with an electrically conductive portion 20 having a plurality of electrically conductive contact positions 25 disposed on a substrate 26 in a contact path in a clock-like fashion. A plurality of insulating contact portions 30 are disposed between contact positions 25. Examples of conventional switch substrates include switch substrates in which the electrically conductive portion is formed of a metal sheet and the insulating portion is formed of insulating synthetic resin. Other conventional switch substrates include an electrically conductive portion that is formed of conductive resin and the insulating portion is formed of insulating synthetic resin.
Three embodiments of a conventional switch substrate 61, 62 and 63 are shown in FIGS. 6, 7 and 8, respectively. Throughout the application, similar elements are assigned the same reference numerals. Switch substrate 61 is formed with an insulating resin layer 8 disposed on a conductive resin layer 7. An electrical contact portion or conductive pattern 2 of conductive resin layer 7 protrudes through the surface of insulating layer 8 and can be contacted by an electrical contact brush 4. Switch substrate 62 is similar to substrate 61, except that a continuity pattern 2' is below the level of insulating contact portion 3 of insulating layer 8. A conductive or continuity pattern 2" of substrate 63 is above the level of insulating contact portion 3 of insulating layer 8.
The conventional switch substrates have several disadvantages. When the switch substrate rotates, portions are worn by friction between the switch substrate and the contact brush. This leads to the accumulation of abrasion powder on the contact brush. The abrasion powder is typically composed mainly of insulating synthetic resin and interferes with electrical contact between the contact brush and the conductive portions. This can lead to problems such as chattering.
In FIG. 6, continuity pattern 2 and insulating contact portion 3 are on the same level. Contact brush 4 is electrically coupled to continuity pattern 2 within the region designated by a double arrow D1 (continuity region). Conductive pattern 2 has a width of a double arrow E which is as wide as D1. It is desirable to make the width of D1 as small as possible to detect small displacements and to detect the greatest number of positions. However, when continuity pattern 2 is formed of conductive resin, the width of continuity pattern 2 cannot be decreased past a practical limit since there is a minimum size filling limit when resin is molded into a conventional metal mold. When continuity pattern 2 is formed of a metal sheet, there is also a practical minimum size limit in forming continuity pattern 2.
FIG. 7 illustrates a switch with a continuity pattern 2' lower than the surface of insulating contact portion 3. Accordingly, the width of continuity region D2 is shorter than width E of continuity pattern 2'. D2 is narrower than D1. Insulating abrasion powder tends to become accumulated over continuity pattern 2' in the depression in the insulation portion 3 thereby leading to continuity failure.
Referring to FIG. 8, continuity pattern 2" protrudes above insulating contact portion 3. This assists in eliminating insulating abrasion powder from contact brush 4 to decrease continuity failure. However, raised pattern 2" causes the width or a continuity region D3 to increase to a width wider than width E of raised continuity pattern 2". It thereby becomes difficult to detect microdisplacements and to detect a plurality of positions.
Conventional switch substrate manufacturing methods also have drawbacks. An electrical continuity portion is formed and positioned in a metallic mold. The insulating portion of the substrate is then molded thereon. A gap tends to occur between the protruding continuity pattern and the metallic mold when the dimensional accuracy and the positioning accuracy are not sufficiently precise. For that reason, the surface of the continuity pattern can become covered by insulating resin. This leads to product defects.
Accordingly, it is desirable to develop an improved switch substrate and method of manufacture which avoids the shortcomings of the prior art.